The present invention generally relates to a method of making bonded multilayer, flat-plate electrochemical cell devices, such as rechargeable batteries and supercapacitors. More specifically, the invention provides a method for establishing persistent interfacial bonding between laminated planar electrode and microporous separator members utilized in such electrochemical devices.
Widely deployed primary and secondary, rechargeable lithium-ion electrochemical cells are typical of electrochemical devices to which the present invention is directed. Such cells comprise layers, or membranes, of respective positive and negative electrode composition members assembled with a coextensive interposed separator member comprising a layer or membrane of electrically insulating, ion-transmissive material. This multilayer electrochemical cell structure is normally packaged with a mobile-ion electrolyte composition, usually in fluid state and situated in part in the separator member, in order to ensure essential ionic conductivity between the electrode members during charge and discharge cycles of the electrochemical cell.
One type of separator for this purpose is a microporous polyolefin membrane, either of single- or multilayer structure such as described, for example, in U.S. Pat. Nos. 3,351,495; 5,565,281; and 5,667,911. When employed as rechargeable electrochemical cell separators, these porous membranes not only effectively retain within their porous structure the essential liquid electrolyte compositions, but they also provide an additional advantage in that they possess an automatic thermal shutdown feature which prevents uncontrolled heat buildup within the electrochemical cell, such as might otherwise result in a dangerous explosive condition, for instance during excessive cell recharging. This built-in safety mechanism relies on the fact that the melting point range of the polyolefins utilized in the fabrication of the separator membranes is at the lower end of the danger zone of electrochemical cell heat buildup. Thus, in the event of a runaway cell heating episode, the porous polyolefin separator membrane becomes heated to a point of melting and its pore structure collapses, thereby interrupting the essential ionic conductivity within the cell and terminating the electrochemical reaction before a dangerous condition ensues.
The packaging of electrochemical cell structures has heretofore regularly taken the form of a metal container, whether, for example, in elongated tubular (cylindrical) or flattened (prismatic) shape, which has commonly been relied upon to not only contain the liquid electrolyte component, but also to impart the significant stack pressure required to maintain close physical contact between the individual cell electrodes and the interposed separator member. This intimate contact, along with the composition of the electrolyte, is, as previously noted, essential to efficient ion transport between electrodes during operation of the electrochemical cell.
More recently, however, the profusion and continued miniaturization of electronic devices powered by Li-ion batteries and similar electrochemical energy storage cells has generated a demand for a greater number of cell package shapes and dimensions, e.g., relatively broad, yet thin, lightweight packages having a significant degree of flexibility. For example, numerous end use applications make thin, flexible tablet-style packages of polymer film more desirable than the prior rigid-walled high-pressure metal can containers. However, these more flexible packages are decreasingly capable of achieving and maintaining the substantial physical pressures required to ensure the noted essential intimate interlayer contact throughout the electrochemical cell.
In order to minimize the deleterious effect of decreased physical stack pressure previously relied upon to establish the necessary contact between electrochemical cell components, developers have progressed to the use of direct adhesive bonding between electrode and separator layers to ensure their essential intimate contact. Typical of such innovations are electrochemical cells utilizing polymer-based electrode and separator members, such as described in U.S. Pat. Nos. 5,296,318; 5,456,000; 5,460,904 and 5,540,741.
In those fabrications, compositions of polymers, such as polymers and copolymers of vinyl chloride, acrylonitrile, methyl methacrylate, ethylene oxide, vinylidene chloride, and vinylidene fluoride, notably of poly(vinylidene fluoride) (PVdF) copolymers with hexafluoropropylene, which are compatible with efficient liquid electrolyte compositions, are utilized as binders in both the electrode and the separator members to not only promote essential ionic conductivity, but also to provide a common composition component in those cell members which promotes strong interfacial adhesion between them within a reasonably low laminating temperature range. Such laminated, multilayer rechargeable electrochemical cells operate effectively and exhibit a stable high capacity and excellent discharge rate performance even though packaged in flexible, lightweight polymeric film enclosures.
Although such laminated electrochemical cells and like energy storage devices have significantly advanced the art in miniaturized applications, the use of substantially non-porous polymeric matrices and membranes in their fabrication has deprived these devices of the desirable thermal shutdown feature achieved when using the microporous polyolefin separator membranes. However, the low surface energy exhibited by the polyolefin membranes renders them highly abherent in nature and thus inhibits their strong, permanent adhesion to many polymeric electrode layer compositions, particularly within a reasonable temperature range which does not lead to melting and, thus, thermal collapse, of the porous structure of the polyolefin membranes.
Some attempts have been made by electrochemical cell fabricators to overcome the adhesion-resistant property of the otherwise desirable microporous polyolefin separator membranes by introducing specifically formulated adhesive polymer compositions into the region of electrode and separator member interfaces, such as described by Abraham et al. in the Journal of Electrochemical Society, vol. 142(3), pp. 683-687 (1995) and in U.S. Pat. Nos. 5,837,015 and 5,853,916. However, it has generally been found that the application of such adhesive compositions, whether by overcoating, dipping, extrusion, or the like, significantly occludes or otherwise interferes with the porous structure of the polyolefin membranes and causes a deleterious decrease in electrolyte mobility and ionic conductivity. Further, the addition of substantial amounts of such adhesive materials increases the proportion of non-reactive components in a cell, thereby detracting from the specific capacity of any resulting energy storage device.
Typical of such attempts to achieve suitable interfacial bonding between electrode and separator cell are the procedures described in U.S. Pat. Nos. 5,681,357 and 5,716,421. There, a layer of PVdF homopolymer is applied to the microporous separator membrane from a solution in organic solvents when the membrane is intended to be employed in the fabrication of an electrochemical cell by thermal lamination with electrodes comprising binder matrix compositions of a similar polymer. It was apparently intended that the added polymer layer would not be of such excessive thickness as to occlude the porosity of the membrane, but rather would provide an intermediate transition in compatibility to the matrix polymer binder of preferred electrode layer compositions. This approach has proven to be insufficient in itself to enable satisfactory interfacial bonding between cell component layers at lamination temperatures below the critical level which results in collapse of separator porosity and its attendant loss of effective ionic conductivity and desirable shutdown capability. Either the added polymer filled the pores of the membrane or the layer was too thin to establish an interfacial bonding of any substance.
In an attempt to overcome this difficulty, a bonding process was devised which involved heating the assembled individual components of a multilayer structure under pressure within a package also enclosing a lithium salt-containing organic electrolyte solution which was to act as a mutual adhesive-forming solvent for the added polymer and the polymer of the electrode compositions. However, this method suffers several problems with respect to assembly and cell performance. First, it is extremely difficult to achieve within an enclosing package a sufficiently controlled and uniform pressure on a multi-ply folded or wound electrode/separator assembly to obtain an adequate strong bond between the respective layers, particularly in the fold region. Second, very thin electrode layers and current collectors have to be used to prevent the electrodes and the current collectors from cracking and delamination. Third, heating a liquid electrolyte activated electrochemical cell to a temperature sufficiently high to effect such bonding is deleterious to the cell""s long-term electrochemical performance and often causes permanent physical and chemical damage to the multilayer foil packaging material and the foil feed-through tabs typically employed in the fabrication of such flat electrochemical cells.
Other methods directed at achieving some measure of bond strength between microporous polyolefin separator and polymeric composite electrode members while preserving the open-pore structure of the separator member have been tried. U.S. Pat. No. 5,981,107 suggests a method in which numerous small dots comprising a fluid adhesive mastic of PVdF in N-methylpyrro-lidinone (NMP) are applied to both sides of a microporous polyolefin separator and the separator is then sandwiched between two PVdF polymer composition electrodes under pressure followed by drying of the applied adhesive. It was apparently intended that the dispersed adhesive pattern would maintain an open-pored field within which electrolyte could freely reside; however, since NMP is a powerful solvent for PVdF and its copolymers, it significantly dissolves the binder polymer in the electrode and causes local filling of the micropores of the separator with a PVdF polymer, thus decreasing the effective ionic conductivity of the separator. In addition, the applied adhesive polymer composition unproductively increases the cell mass, thus lowering its effective energy storage capability.
U.S. Pat. No. 6,024,773 discloses a similar method which involves uniformly coating both sides of a separator member with a fluid solution of PVdF in NMP or other strong solvent, sandwiching the separator between electrode members, pressing the three layers together, and drying the assembly at elevated temperature to form a laminate. The problems mentioned above are even more pronounced in this method.
Therefore, there remains a need in the art for an improved and economical method of fabricating high-capacity, thermal shutdown-protected, electrochemical cells incorporating microporous polyolefin separator membranes. There also remains a need for a simple, economical, and easily controlled method of effectively bonding microporous polyolefin separator membranes into high-capacity, high-discharge rate, shutdown-protected, bonded-electrode rechargeable electrochemical cells.
The present invention provides a simplified method of fabricating flat, high-capacity, high-discharge-rate, thermal-shutdown-protected electrochemical cells through the use of polymer matrix electrodes and economical, commercially available microporous polyolefin-separator membranes. More particularly, the present invention comprises a method for facilitating the lamination of electrochemical cell members without resort to additional polymeric adhesive compositions and at laminating temperatures and pressures which effect firm interfacial bonding between polymer matrix electrode members and an unmodified microporous separator membrane, yet are sufficiently low to avoid thermal and mechanical collapse or other occlusion of the porous membrane structure of the cell separator member.
In the method of the present invention, positive and negative electrode members are provided which respectively comprise layers of polymeric matrix compositions of active electrode materials, such as Li-ion-intercalatable carbons and transition metal oxides, e.g., LiCoO2 and LiMn2O4. Such electrode compositions, preferably comprising poly(vinylidene fluoride) polymers or copolymers, are typically highly compacted or densified layers, such as formed under calendering or laminating pressure, and may additionally be coated upon or laminated into sub-assemblies with solid or reticulated metal foil current collector members.
A novel complementary separator member is prepared which comprises a common, commercially available thermal-shutdown-capable porous membrane consisting essentially of one or more microporous layers of polyolefin into which, according to the invention, there has been deposited a desired amount of a primary plasticizer for the electrode matrix polymer. The amount of primary plasticizer introduced into the microporous separator member may be readily controlled by applying to the separator member by any convenient means, such as coating, immersion, or spraying, a predetermined concentration of the plasticizer in a volatile solvent vehicle. The appropriately diluted solution of plasticizer is absorbed into the pores and, following simple evaporation in air to remove the volatile solvent, the plasticizer is deposited in the pores of the separator.
The resulting treated separator member is interposed between the electrode members in contact with the surfaces of the polymeric compositions, and the assemblage is heated under pressure in common laminating apparatus, such as comprise heated rollers or platen presses, to effect fabrication of the electrodes and separator composite into a unified, flexible electrochemical cell structure. During the laminating operation, the pressure applied to the cell member assemblage forces the plasticizer from within the separator pores and into contact with the contiguous surfaces of the electrodes where, in part accelerated by the applied laminating heat, the interfacial region of the electrode composition matrix is softened by the plasticizer to enable adhesion of the composition to the contacting separator member surface. By virtue of this unique aspect of the invention, the laminating temperature may be maintained safely below the thermal shutdown threshold of the microporous membrane, yet the laminated adhesion between the electrode and separator surfaces is sufficient to withstand the rigors of cell cycling and usage, such adhesion often exceeding the cohesive strength of the electrode compositions.
Following the lamination of the cell members, the plasticizer provided by the separator, as well as such plasticizer as may have initially comprised the electrode polymer matrix composition, may be removed by liquid or supercritical-fluid extraction or by simple evaporation prior to packaging the resulting multilayer bonded cell into a flexible pouch or envelope with a measure of a lithium salt-containing electrolyte solution in order to activate the cell.
The plasticizer comprises about 10% to 30% of the separator-treating solution, preferably about 15% to 20%. Useful plasticizers are moderately volatile and include alkylene carbonates, dialkyl phthalates, dialkyl succinates, dialkyl adipates, dialkyl sebacates, trialkyl phosphates, polyalkylene glycol ethers and mixtures thereof, a preferred plasticizer being propylene carbonate (PC). The vehicle solvent is selected from organics which are significantly more volatile than the plasticizer in order to enable its removal from the separator member without excessive heating or other treatment. Lower alcohols, ketones, esters, aliphatic hydrocarbons, halogenated solvents, such as chlorinated hydrocarbons, chlorinated fluorocarbons, and mixtures thereof are useful in this respect.
Electrode members may be in the form of highly densified polymeric electrodes deposited on or laminated to metal-foil current collectors, such as those used in liquid-electrolyte Li-ion cells, or densified and non-extracted or extracted plastic Li-ion electrodes, such as those disclosed in U.S. Pat. Nos. 5,418,091; 5,429,891; 5,456,000; 5,460,904; 5,540,741; 5,571,634; 5,587,253; and 5,607,485; wherein preferably at least one electrode has a reticulated metal current collector in the form of an expanded-metal grid, mesh, metallic non-woven material, etched foil or perforated foil.
Lamination of the electrode members with a separator member treated to include plasticizer according to the present invention is preferably carried out between heated pressure rollers at a temperature and pressure level, now made sufficiently low by the inventive treatment, which does not significantly affect the porous structure, i.e., a temperature below the shutdown temperature of the separator membrane. Effective lamination may be carried out between 70xc2x0 C. and 13xc2x0 C., preferably between 100xc2x0 C. and 125xc2x0 C., and more preferably at about 110xc2x0 C., and with a linear load between about 20 and 180 kilograms per centimeter (kg/cm), preferably between about 55 and 125 kg/cm, although it should be apparent to the skilled artisan that the optimum temperature and pressure conditions will depend on the particular laminator construction and mode of its use.
The adhesive bond formed at the electrode and separator member interfaces as a result of the present invention was found to be surprisingly durable despite the fact that the normally abherent polyolefin surfaces of the microporous separator had not been previously subjected to expensive pre-coatings or polymeric adhesive compositions. Particularly noteworthy is the fact that the interfacial bonds of these cell members are able to survive extended exposure to solvent-based cell electrolyte compositions even at battery storage temperatures higher than about 80xc2x0 C. The surprising efficacy of separator-borne plasticizer alone in establishing strong interfacial cell member bonds provides a novel and simplified means for making long-sought-after, permanently bonded, flat rechargeable electrochemical battery cells with excellent performance characteristics and long operating life.